1. Field of Invention.
This invention relates to a method of producing an exhaust valve for a Diesel engine, and more particularly, to such a method wherein the contact surfaces between the valve and seat are formed by a coated layer of a mixture of ceramic grains and metal grains, with the proportion of ceramic grains in the mixture being greater toward the surface than toward the valve and seat bodies.
2. Discussion of Prior Art.
Diesel engine exhaust valves now used are easily burned by exhaust gases, especially in the middle speed and high speed Diesel engines which exhaust gases of high temperatures, and more especially when inferior or lower grade fuel is used. Burning occurs of the surfaces which form the contact point between the valve and the seat. The exhaust gases contain oxides of low melting point, such as V.sub.2 O.sub.5 or Na.sub.2 SO.sub.4, which penetrate into the contact surfaces and cause oxidation which is accelerated by high temperatures. This is referred to as the blowing and burning phenomena.
A conventional means of dealing with the problem is to use a Cr-heat resistant steel or Ni-based super heat resistant alloy for the valve body and seat body, with a portion of the body at the contact surfaces being prepared with weld padding or coat padding of corrosion resistant alloy of base of Co or Ni having a high hardness factor (e.g. Hv 600 to 700). However, when fuel is inferior, the contact surfaces are usually instantly damaged by the blowing and burning phenomena, since it is only padded with the corrosion resistant alloy.
Another approach has been to make the contact surfaces of a coated layer of Co or Ni based alloy having ceramic grains uniformly dispersed therein. But, this was found to be poor in durability to repeated shocks. Also, such a coated layer of alloys having ceramic grains uniformly disposed therein is of low density and the layer is of low melting point, which accelerates oxidation at high temperatures, so that the blowing and burning phenomena still exists.
Thus, in the art, there still exists a deficiency in that no method exists for the preparation of exhaust valves for a diesel engine wherein the contact surfaces between the valve and valve seat have such desired properties as thermal shock resistivity, toughness, resistance to cracks, resistance to exfoliation, and corrosion resistance, and elimination of the blowing-burning phenomena.